The disclosures herein relate generally to computer systems and more particularly to a printed circuit assembly having a conductive pad array with in-line via placement. Printed circuit assemblies such as rigid printed circuit boards and flexible printed circuits, are widely used components in all types of electronic devices. For example, printed circuit assemblies are commonly used as motherboards or other types of system boards in computer systems. Printed circuit assemblies provide for dense routing of electrical signals and power, and are relatively inexpensive to fabricate.
The size of electronic components continues to decrease and circuit routing densities continue to increase. Accordingly, it is necessary to increase the density of printed circuit assembly components, such as interconnect pads, conductive vias, and traces. Increasing the density of the printed circuit assembly components requires spacing such components closer together without increasing the occurrence of short circuits between adjacent printed circuit assembly components.
U.S. Pat. No. 5,875,102 discloses a ball grid array integrated circuit package which has a plurality of conductive vias connected to a plurality of solder pads located on a bottom surface of a package substrate. Each conductive via has a portion located within a solder pad to increase the routing space of the substrate, and a portion located outside the solder pad to allow outgassing from the conductive via. The bottom surface also has a solder mask which covers the conductive vias and contains a number of holes that expose the solder pads. The holes allow solder balls to be connected to the solder pads. The solder balls can be reflowed to attach the package to a printed circuit board. This type of package configuration increases routing space, but is relatively expensive to fabricate.
U.S. Pat. No. 5,706,178 discloses a ball grid array integrated circuit package which has a plurality of elliptically shaped solder pads located on a bottom surface of a package substrate. The bottom surface also has a solder mask which contains a number of holes that expose the solder pads. The holes allow solder balls to be attached to the solder pads. The solder balls can be reflowed to attach the package to a printed circuit board. The elliptically shaped solder pads have a width that is smaller than the length of the pads. The narrow width portion of each solder pad provides additional routing space on the bottom surface of the package. The solder mask hole diameters are less than the length of the solder pads so that portions of the solder pads are anchored beneath the solder mask. The anchored portions of the solder pads increase the solder pad peel strength.
U.S. Pat. No. 5,585,162 discloses a flexible circuit construction that allows solder balls to be mass reflow attached to the ground plane of a double-sided flexible circuit. The circuit includes a first conductive via which is separate from the remainder of the ground plane. The first conductive via is electrically connected to the ground plane through a second conductive via. The second conductive via is positioned at a distance from the first conductive via by a circuit trace on the side of the flexible circuit opposite the ground plane.
In high density printed circuit assemblies, the distance between one printed circuit assembly component relative to another is decreased. By rearranging selected printed circuit assembly components, additional space between such selected components is provided. The additional space can be used for adding more printed circuit assembly components such as interconnect pads and conductive vias, or for routing additional traces or larger traces between the selected components.
Conventional printed circuit assemblies that provide additional trace routing space are known and are discussed herein below. These conventional printed circuit assemblies provide additional routing space at the expense of reliability and manufacturability. Each conductive via and adjacent non-connected interconnect pad are susceptible to shorting during reflow of a connected interconnect element, such as a solder ball. In some of these conventional printed circuit assemblies, each conductive via is centered between two interconnect pads. Because of the close proximity of each conductive via relative to the adjacent non-connected interconnect pad, the potential exists for one or more of the conductive vias to be inadvertently connected to the adjacent non-connected interconnect pad during a solder reflow operation.
Therefore, what is needed is a low-cost and reliable printed circuit assembly that provides additional space for routing wider traces or an increased number of traces through an array of conductive vias and interconnect pads.
Accordingly, in one embodiment, a printed circuit assembly includes a conductive via that provides additional space for routing traces without sacrificing reliability or manufacturability. To this end, a printed circuit assembly includes a circuit substrate having spaced apart first and second major surfaces. A first and a second interconnect pad are spaced apart on the first major surface of the circuit substrate. The first and the second interconnect pads are positioned on a reference axis. The first interconnect pad is spaced apart from the second interconnect pad by a first distance. A first and a second conductive via extend through the circuit substrate between the first and the second major surfaces. The first and the second conductive vias are positioned adjacent to and electrically connected to respective ones of the interconnect pads. Each one of the conductive vias is positioned on the reference axis and is spaced apart from the respective interconnect pad by a second distance. The first distance is substantially greater than the second distance wherein each conductive via is offset toward the connected interconnect pad.
A principal advantage of this embodiment is that an array of conductive vias and an array of interconnect pads can be configured to provide increased space for routing traces therethrough.